walker: let walker_say take arbitrary formats

walker: let walker_say take arbitrary formats

We take a printf-style format and a single "char *"
parameter, and the format must therefore have at most one
"%s" in it. Besides being error-prone (and tickling
-Wformat-nonliteral), this is unnecessarily restrictive. We
can just provide the usual varargs interface.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>